We have observed the expression of TIMP-1 at the RNA level in hyperplastic tonsils, activated T-cells and some Burkitt cell lines. TIMP-1 was not expressed by numerous T-cell lines or B-cell lines derived from low-grade lymphomas. We have demonstrated secretion of TIMP-1 protein by these Burkitt cell lines and observed that TIMP-1 expression correlates with cell proliferation rates. We are currently studying the effect of TIMP-1 on the kinetics of cell proliferation in the Burkitt cell lines as well as the aggressiveness of these lines in nude mice. We are undertaking a study of the expression of TIMP-1 in aggressive and low-grade B-cell neoplasms using antibodies against TIMP-1.